When gear executes a command and passes it a pkg.tar generated from a tag
object,
it should pass to this command a SOURCE_DATE_EPOCH environment variable
initialized from the tag object: if the tag object is signed, SOURCE_DATE_EPOCH
should be defined to the signature time, otherwise to the time saved in tagger
entry.

Alternatively, when given a tag object, gear can ignore the signature time and
always use the time saved in tagger entry, assuming that in signed tags the
time saved in tagger entry is also signed.
In addition, when given a commit object, gear can use the time saved in
committer entry.

I'm going to implement the following change:
When gear extracts from a tag object, it would set $SOURCE_DATE_EPOCH
to the tagger timestamp before invoking an external command.
Likewise, when gear extracts from a commit object, it would set
$SOURCE_DATE_EPOCH
to the committer timestamp before invoking an external command.
When $SOURCE_DATE_EPOCH is set, gear-command-tar would specify tar --mtime
option,
thus setting mtime of files in the pkg.tar to $SOURCE_DATE_EPOCH.